With this type of power supply, it is usually desirable to deliver electrical energy to the useful load CU of the power supply at a constant power level. Generally, the useful load consists of a load per se connected in series with a preregulator constituted by a "BUCK" or "BOOST" circuit. This type of circuit is essentially reactive and hardly consumes any power at all in normal operation. The characteristic curve of constant power consumption in the I-V plane, where I is the current fed to the useful load and V is the voltage supplied to the useful load, is hyperbolic in shape as shown by the dashed line in FIG. 1. The same graph also shows the current/voltage (I/V) curves for a solar cell 1 (continuous line), and for a buffer storage battery 2 plus diode D (dot-dashed lines), thereby enabling a composite characteristic curve to be defined (dotted line 1) for the assembly comprising the solar cell 1 and the buffer battery 2. The useful load may thus be fed at constant power at three operating points A, B, or C where the composite characteristic of the solar cell 1 and its battery 2 intersects the constant power hyperbola. Because of the nature of BUCK or BOOST circuits, and mainly because of the difference in absolute value between the slope of the composite characteristc (dots 1) and the constant power hyperbola, operating point B is inherently unstable. Any variation in voltage and/or current when the system is operating at point B has the effect of bringing the real operating point either to point A or else to point C along the constant power hyperbola. Points A and C are inherently stable with the sign of the difference between the hyperbola slope and the composite slope being opposite to the sign of the same difference at point B.
However, it is undesirable for the system to operate at I.sub.A, V.sub.A since in this state the current supplied by the battery is greater than the current supplied by the solar cell, thereby regularly discharging the buffer battery. The solar cell is capable of supplying all of the power required only at operating point I.sub.C, V.sub.C.
Voltage regulators for regulating the voltage delivered by such systems may be used in order to mitigate the inevitable fluctuations or variations in the supply voltage due either to variations in the illumination of the solar cell or else to variations in the charge and/or internal resistance parameters causing variations or fluctuations in the corresponding operating point. Such regulators are described in the patent filed in Belgium under the No. 853 124 and in the name of the Organisation Europeenne des Recherches Spatiales (i.e. the European Space Research Organization). These voltage regulators subdivide the solar cell into a plurality of elementary solar cells, thereby quantifying the power delivered to the load, and regulating said consumed power between two successive quantification levels. Although this type of device regulates to power satisfactorily, it does not allow the operating point of the system to be adjusted to one of the desired points, and overall it resembles a voltage limiter.
Preferred implementations of the present invention remedy the above drawbacks.